What really happens when you plug a speaker port/vent?

[RickS]

As an extension of my comparison test of speaker damping materials, thought I would see how plugging the vent affects its output. In this case, for the Dayton C-Note speaker. As you will see, the result depends a lot on what material you use...

So, the red trace is the baseline speaker as supplied by Dayton. Test setup is a UMIK-1 mic centered on the port at 0 meters. My first "plug" was some light Acousta-Stuf along the length of the port as shown in green. As you can see, even a light application, significantly stunted the bass output of the port. However, the same treatment still left a major portion of the port resonance around 800 Hz.

Next, I tried an open cell foam plug (as supplied by Mirage for Omni 150s). Surprisingly, the plug measurements (blue trace) were very comparable to the Acousta-Stuf application. It still left some major port leakage. Finally wanted to show the result if you densely stuff the port. In this case (orange trace) I rolled a plug out of F13 felt and jammed it tightly into the port. As shown, except for around 250 Hz, this seriously tamped down on the port output.

In conclusion, you can get varying results on how you plug a port. A simple cell foam plug does flatten the bass well, but if you really want to ensure no ugly output occurs (particularly if it is a front facing port), need to ensure you stuff something dense into the port.

 

[DonH56]

What I find interesting is the impact on the upper frequencies, implying significant internal coupling and modulation from the port on the midrange.

 

[AnalogSteph]

I wouldn't have expected that the internal resonances would actually be harder to get rid of than the main bass output. Pressure vs. velocity may be different between the two. Good to know. Makes you wonder whether it's possible to make an external absorber that lets bass pass but effectively attenuates the midrange components... although it would be prudent to address the internal stuffing first, of course.

 

[RickS]

I wouldn't have expected that the internal resonances would actually be harder to get rid of than the main bass output. Pressure vs. velocity may be different between the two. Good to know. Makes you wonder whether it's possible to make an external absorber that lets bass pass but effectively attenuates the midrange components... although it would be prudent to address the internal stuffing first, of course.

As I show elsewhere, shortening the port removes the midrange nasties well. The little woofer is not suited for much low bass output and would be better off adjusting via eq or a tone control imo.

 

[Juhazi]

I measured my speaker in 2015. Plug is the factory-made foam plug. Block was a pair of cotton socks IRC. Fortunately the port is on the backside. I still use these as HT rear speakers, with plugs.

 

[Salt]

What I find interesting is the impact on the upper frequencies, implying significant internal coupling and modulation from the port on the midrange.

That's why to arrange the outlet there where This does matter the less.

 

[Salt]

Well, problably only the black sock of the big foot with audiophile sweat, compressed (hmph) into the 'resonator' may be the most suitable (acoustical, not olfactorial)
Specimen can be provided by PM

 

[SIY]

One other interesting and diagnostic measurement is the changes in impedance.

 

[Salt]

If plugged correctly it should change from two to one max.

 

[RickS]

One other interesting and diagnostic measurement is the changes in impedance.

I agree as except for the extreme case, did that here…

Understanding Vented Speaker Impedance

As some of you are aware, have been playing around with my Purifi SPK5 parts and working to determine a final configuration for them. As I have been trying various vented alignments, reminded me that I planned to share some useful observations about speaker impedance measures. I am multitasking...

www.audiosciencereview.com

 

[RickS]

Well, problably only the black sock of the big foot with audiophile sweat, compressed (hmph) into the 'resonator' may be the most suitable (acoustical, not olfactorial)
Specimen can be provided by PM

Can keep that specimen in Germany thanks!

 

[elshaddai]

When I was playing around with plugging rear ports, I found some rubber bottle stoppers that fit perfectly. I wonder how those would measure...

 

[RickS]

When I was playing around with plugging rear ports, I found some rubber bottle stoppers that fit perfectly. I wonder how those would measure...

Depends on how thick the rubber is. The other issue can be how good a seal is made as any plug is likely to be subject to loosening over time. Air leaks will change design performance and can add noise. Ideally if the port can be easily removed, would plug the hole rather than the port. Another approach would be to plug the port from the inside as this would lessen the pressure on the plug.

 

[elshaddai]

Depends on how thick the rubber is. The other issue can be how good a seal is made as any plug is likely to be subject to loosening over time. Air leaks will change design performance and can add noise. Ideally if the port can be easily removed, would plug the hole rather than the port. Another approach would be to plug the port from the inside as this would lessen the pressure on the plug.

These are the ones I found: https://www.amazon.com/gp/product/B0BN2W3ZGH/ Inserted about 2/3rds into the port hole. Nice fit, can't imagine much leaking around it.

 

[JeremyFife]

Just had an odd thought: can you *extend* a port ... stick a tube or similar into it. Would there be any reason for doing that.

Not planning to, just suddenly curious.

 

[RickS]

These are the ones I found: https://www.amazon.com/gp/product/B0BN2W3ZGH/ Inserted about 2/3rds into the port hole. Nice fit, can't imagine much leaking around it.

Those do look robust. If you regularly play speakers loudly, would check to see if they are staying tight. Otherwise, would not be concerned.

 

[RickS]

Just had an odd thought: can you *extend* a port ... stick a tube or similar into it. Would there be any reason for doing that.

Not planning to, just suddenly curious.

Yes, recall lining the port had that effect. See the link in post #10.

 

[RickS]

Btw, thought it interesting to know how well my felt plug sealed the port. So rather than aim the mic at the port, aimed it at the baffle just below it. The result was almost identical to the port measure for the felt plug. My takeaway from this is that the plug sealed the port well.